This invention relates to pulse width measuring circuits and more particularly to a circuit for producing an analog voltage proportional to the pulse width of an input pulse train.
There are many requirements to measure the pulse width of pulses of a pulse train. For example, U.S. patent application Ser. No. 659,807, filed concurrently herewith, discloses a low cost pulsed radar altimeter for determining when an object has reached a predetermined altitude. This altimeter utilizes a single oscillator which serves as a transmitter and a superregenerative receiver and indicates the predetermined altitude by denoting when the pulses from the oscillator increase in width. Applicants of that application found that rather than measuring pulse width directly they instead measured the duty factor of the pulse train from the superregenerative receiver. The duty factor of a pulse train is precisely related to pulse width when the pulse repetition frequency and amplitude are constant.
For measuring moderate duty factors a simple lowpass filter can be used. Its output is the average voltage, which is the product of pulse amplitude and duty factor. However, for the low duty factors typically employed in radars (0.001 or less), such a circuit has a very low output.
Assuming a TTL waveform with an amplitude of four volts and a duty factor of 0.001, the output of a lowpass filter will be four millivolts. If it is desired to detect a one percent change in duty factor, that would be forty microvolts. Typical comparators have on the order of one millivolt drift and d.c. amplifiers are no better. Therefore, one might be required to use something like a chopper stabilized amplifier at great expense.
Accordingly, it is an object of this invention to provide an improved duty factor measuring circuit.
It is another object of this invention to provide a circuit for converting pulse width to an analog voltage.
It is further object of this invention to provide a simple, low cost, passive circuit for producing an analog voltage proportional to the duty factor of an input pulse train.
It is yet another object of this invention to provide a low-loss, duty factor measuring circuit.